Duplication of chromosomes without the division of nucleus is called as

The correct option is C Endomitosis
Endomitosis involves the duplication of chromosomes without the division of nucleus. Endoreplication or endomitosis is a replication of the nuclear genome in the absence of cell division, which leads to elevated nuclear gene content and polyploidy. Endoreplication can be understood simply as a variant form of the mitotic cell cycle (G${}_1$-S-G${}_2$-M) in which mitosis is aborted prior to cytokinesis or circumvented entirely, due in part to modulation of cyclin-dependent kinase (CDK) activity.

So, the correct answer is option C.